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Say cheese! NASA's Cassini spacecraft will capture a rare image of Earth early tomorrow morning as it photographs Saturn backlit by the Sun.

In December 1968, the crew of Apollo 8 emerged from behind the Moon and captured one of the earliest images of the Earth rising above the lunar horizon.

Since the iconic Earthrise image was taken, several spacecraft have captured our home planet as a blue ball floating in the blackness in space, but there are very few images of Earth from the outer solar system.

The new image will join just other two images from our outer solar system -- the 'Pale Blue Dot' image taken six billion kilometres away from Earth by Voyager 1, and a 2006 image of Earth from Saturn, also taken by Cassini.

Dr Carolyn Porco was part of the Voyager team when the spacecraft took the Pale Blue Dot image, and is currently leader of the Cassini Imaging Team. She played a role in capturing both images.

"It's a very visceral, powerful recognition that everyone feels when they see us as just a star-like dot in the distance. Like, 'So that's what we would look like to another world'," says Porco.

But this time, Porco wants the world to know it is being photographed.

"My feeling was that it was a missed opportunity (in 2006) not to let everyone know in advance that their picture was being taken."

Porco is urging Earthlings to wave at Saturn at the time the photograph is being taken at approximately 7.37 am 20 July AEST (21:27 19 July UTC).

"'The Day the Earth Smiled' is the name I've given to the day that Cassini will be imaging the Earth from Saturn. [It] marks the first time the Earth's inhabitants know in advance that our image is being taken from a billion miles away," she explains.

"I want it to serve as an opportunity for all the world to contemplate our cosmic place and to appreciate not only the beauty of our planet and the life it sustains; but also the magnitude of the achievements that have made this interplanetary photo session possible."

Unfortunately, for many people around the world -- including Australia -- Saturn won't be visible in the sky when Cassini takes Earth's picture.

Even though Saturn will be below the horizon in Australia, amateur astronomer Ian Musgrave urges people to get involved.

"Earth will be just a dot, so it doesn't really matter if you are waving at the ground, it's getting out there and participating in a historic moment, one of the handful of portraits of Earth taken from a distant spacecraft," says Musgrave.

"And if we have a whole pile of Australians waving at the ground, that will be unique."

Porco says the best image submitted to their website will be included in a 'Message to the Milky Way' transmitted from the Arecibo radio telescope in Puerto Rico.

More science to come

Taking pictures of Earth from afar isn't the only thing that Cassini has been doing since its launch in 1997.

Thousands upon thousands of images of Saturn, along with its rings and more than 60 moons have been beamed back to Earth.

As well as carrying the European Space Agency's Huygen probe to the Saturnian moon Titan, Porco says the spacecraft has made some amazing findings including the discovery of a salty, organic-rich liquid water under the south pole of the moon Enceladus.

"If you want to find out as soon as possible if we have life elsewhere in our solar system, that's the place to go."

According to Porco, Cassini will continue its orbital mission until at least 2017 - 13 years after it arrived at Saturn.

Hello Earth!

Cassini is not the only spacecraft that will capture images of Earth this weekend. NASA's Messenger will take photos of Earth from Mercury tomorrow and Sunday night at 21:49, 22:38 and 23:41 AEST (11:49, 12:38 and 13:41 UTC).

While Mercury high in the daytime sky over Europe, the Middle East and Central Asia, it will be below the horizon in Australia.

Both images will join the iconic views of our planet such as:

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